1. Field of the Invention
The present invention relates to an improved light shield for selectively shielding light sensitive elements in a visible sensor semiconductor device from incident light.
2. Description of Prior Art
Semiconductor devices often comprise light sensitive elements of diverse types made with signal processing circuitry in the same semiconductor substrate. For example, solid state imaging devices can contain detectors and signal processing circuitry on the same silicon chip. The material used in both detectors and the signal processing circuitry is of a photosensitive nature. Incident light that would be essential for the operation of the detectors would be the cause of false signals due to the corruption of the device's output by light absorption in the signal processing circuitry.
Because the light sensitive elements are in close proximity to the signal processing circuitry, special shielding must be built into the device to produce the required selective light shielding. Light must be blocked from striking the signal processing circuitry while allowed free access to the detectors.
In principle, light shields constructed of metal or photoresist could be extremely effective in limiting light exposure to the photodetectors of the semiconductor. However, inherent problems occur with both light shield types now used throughout the industry.
The significant amount of processing circuitry on modern solid state sensors requires a proportional amount of shield overlay to block incident light from the light sensitive processing circuitry. It has not been uncommon to encounter electrical shorts between the metallic light shield layer and the underlying shielded circuitry. This three-fold reduction in a device yield from a wafer due to shorts between the metallic shield and the signal processing circuitry is an expensive problem directly related to the use of metallic light shields. Elimination of the shorting problem while still retaining the basic light shield function would of course result in a significant cost benefit to the manufacturing sector.
A patent which describes a light shield and its use is U S. Pat. No. 3,969,751, "Light Shield for a Semiconductor Device Comprising Blackened Photoresist", dated July 13, 1976, to Israel Drukaroff. This patent taught an opaque layer of material nontransmissive to light, i.e., blackened photoresist, blackened metal or plastic containing particles of black carbon. Photoresist and plastic blackened materials should indeed resolve the problem of metal light shield of shorting between the shield and the processing circuitry. However, both photoresist and plastic containing blackened carbon exhibit instabilities which are accelerated when the layers are exposed to light over long periods of time. This photodegradation, i.e., cracking and peeling is deleterious to the continued effective performance of a light shield.